Fossil fuels such as petroleum, natural gas, coals (particularly bituminous) and nuclear energy source (which are sometimes categorized as fossil fuels) have been heretofore utilized as energy sources. Additionally, global population growth and industrial progress have led to a rapid growth in energy consumption. Among these fossil fuels, coals are extremely abundant in terms of recoverable reserves, and are the most stable supply source. Up until recently, bituminous coals have been conventionally used among the coals. In the future, utilization of the low-grade coals (poor in transport and energy efficiency), such as lignite or subbituminous coals, which account for about a half of the coal resources, will become an important target. Sunbelt regions such as those in Australia, Southeast Asia and the United States of America are abundant in low-grade coals. Further, utilization of biomass such as wood will also become an important target. In order to make the effective use of these unutilized energies (low-grade coals or biomass), there can be utilized a concentrated solar radiation (renewable energy) to perform thermal decomposition and/or gasification to produce hydrogen, carbon monoxide, methane, etc., leading to the creation of a new type of energy source. The resultant gas mixture of hydrogen and carbon monoxide will become a raw material of hydrocarbon fuels such as kerosene, light oil, diesel oil, gasoline, dimethyl ether (DME), methanol, and etc. Additionally, methane is already in commercial use as a clean fuel.
Technical development is actively promoted for a technique of producing hydrogen, etc., through dissociating water by mean of high-temperature solar heat received from a concentrated solar light (see, e.g., Patent document 1). Unfortunately, as the method needs a use of a clear quartz plate as a light aperture (window) of the concentrated solar radiation, such method is not applicable to a thermal decomposition reaction of coals that generate tar or soot.
In recent years, technical development of utilizing a concentrated solar radiation for thermal decomposition of cokes have been pursued (see, e.g., patent document 2), but this technique is applicable only where no tar or soot be generated.
Further, a new attempt at heating the air, by means of concentrated solar radiation, to be fed to a gas turbine is also being made (see, e.g., non-patent document 1). Unfortunately, this technique takes no measures for the heat leakage caused by the reflection and/or re-radiation of concentrated solar radiation that is irradiated upon a heat receiver. Also, on the periphery of this heat receiver is arranged components made of form material having extremely poor thermal transference and absorptivity.
Here, FIGS. 14 to 17 illustrate examples of conventional reaction apparatuses.
FIG. 14 illustrate a system having a multitude of heliostats tracking the movement of the sun and a beam-down light collecting system. The system collects solar lights and then guides the collected solar lights to a reaction apparatus, while introducing water vapor to the reaction apparatus to produce hydrogen by means of two-step thermochemical water-splitting cycle using metal oxide such as iron oxide contained in the reaction apparatus. This two-step thermochemical water-splitting cycle alternately switches from a step of producing hydrogen through a chemical reaction of water vapor and metal oxide (thermochemical water-splitting reaction at the temperature of 900° C.) to a step of performing reductive reaction of metal oxide using, e.g., nitrogen gas (thermal reduction reaction at reaction temperature of about 1400° C.) in a repetitive manner.
FIG. 15 illustrate a system having a multitude of heliostats tracking the movement of the sun and a towered light collecting system. The system collects solar lights and then guides the collected solar lights to a reaction apparatus, while introducing water vapor to the reaction apparatus for producing hydrogen by means of two-step thermochemical water-splitting cycle using metal oxide such as iron oxide contained in the reaction apparatus. This apparatus has a configuration of horizontally laying the apparatus as illustrated in FIG. 14
FIG. 16 illustrates a system that guides collected solar lights to a reaction apparatus while introducing coke and sand through an upper sidewall, and then introduces water vapor through a bottom portion of the reactor for producing hydrogen by means of thermal decomposition reaction. This system is an example of the application of a thermal decomposition of coke in art operation condition where no tar or soot is to be generated.
In an example as illustrated in FIG. 17 (a), solar lights collected from a multitude of heliostats are guided to a heat receiver at various angles. This heat receiver is made of heat resisting material, particularly of Inconel, alumina, silicon carbide, or the like when being used in a condition of higher temperature, or of stainless steel when being used in a condition of a lower temperature. The concentrated solar radiation is partially leaked to the outside of the heat receiver through reflection and/or re-radiation. This Figure illustrates the way how light beams are reflected within the heat receiver when the depth and diameter of the heat receiver are set to be about the same length where incident angles of the collected solar lights, or elevation angles (α) with respect to the central axis of the heat receiver, are respectively set to be 10°, 20°, 30°, 40° or 50° degrees. The number of reflections of a light beam within the heat receiver is as small as 1 to 3, showing a large amount of heat radiation loss.
FIG. 17(b) illustrates the way how light beams are reflected within the heat receiver when the depth of the heat receiver are set to be about twice the diameter thereof where incident angles of the collected solar lights, or the elevation angles (α) with respect to the central axis of the heat receiver, are respectively set to be 10°, 20°, 30°, 40° or 50° degrees. The number of reflections of a light beam within the heat receiver is as small as 2 to 6, implicating a large amount of heat radiation loss.
FIG. 18 illustrates an example of a conventional heat storage system by means of solar light collection. Such heat storage system can be categorized as, a sensible heat storage system (liquid: oil, solids: cement, solid particle, etc.), a latent heat storage system (molten salt, etc.), or a chemical heat storage system. These heat storage system are designed to be operable when being exposed to the solar light, as well as when not being exposed to the solar light (e.g., when solar light is shut out by clouds, or during night-time). Heat storage capacity of the system depends on operating periods during which the system is not exposed to the solar light. Note that this conventional example relates to a sensible heat storage system of solid particle and a chemical heat storage system. The reactant material in the chemical heat storage is in a solid particle state. For this reason, the sensible heat storage system of solid particle and the chemical heat storage system are similar to each other. Here, within the heat receiver is filled with honey comb structures (or foam). That's because solar light cannot directly heat the air. Consequently, the collected solar lights first heat honey comb structures, and the honey comb structures subsequently heat the air. These honey comb structures have a smaller surface area, narrow passages and a small heat transfer rate, which make it difficult to heat the air in a rapid manner.
As illustrated in FIG. 18 (a), under the solar light, the concentrated solar radiation passes through the quartz plate (window) to the heat receiver, causing a low temperature air, fed to the heat receiver, to be heated to a high temperature via the honey-comb structures, forcing the heated air mass to be streamed in parallel through a steam generator and a heat storage tank. The heated air mass fed to the steam generator heats water, generating steam, thus causing the air mass to be in a lower temperature, which is then circulated in the heat receiver. The resultant vapor generates electricity by means of a steam turbine and a generator. The high-temperature air mass fed to the heat storage tank will then pass through small gaps in-between metallic oxide particles, thus producing laminar flow and reducing the a heat transfer rate to be small. The high-temperature air mass slowly conducts heats to heat-storage particles, causing the air mass to be in a lower temperature, which is then circulated in the heat receiver. The metallic oxide particles thus heated within the heat storage tank emit oxygen through a chemical reaction, thereby storing thermochemical heat. In this way, the metallic oxide particles store both sensible heat and thermochemical heat.
As illustrated in FIG. 18 (b), when not being exposed to the solar light, a valve will be switch thereto to feed the air to the heat storage without passing through the heat receiver, thereby rendering the metallic oxide particles within the heat storage tank to be chemically reacted with oxygen in the air, releasing heat, thus heating the air. The heated air mass fed to the steam generator heats water, generating steam, causing the air mass to be in a lower temperature, which is then circulated in the heat receiver. The resultant vapor generates electricity by means of a steam turbine and a generator. Note that this conventional art employs a large heat storage tank. For this reason, it takes a large amount of time to switch from the heat storage mode to the heat releasing mode, thus making it difficult to be adapted in a cloudy weather.